The St. Francis Dam was a curved concrete gravity dam, built to create a large regulating and storage reservoir as part of the Los Angeles Aqueduct. It was located in San Francisquito Canyon (about 40 miles northwest of Los Angeles, Ca. and approximately 10 miles north of Santa Clarita).

The dam was designed and built between 1924 and 1926 by the Los Angeles Department of Water and Power (back then it was the Bureau of Water Works and Supply). The department was under the supervision of its General Manager and Chief Engineer, William Mulholland.

Minutes before midnight on March 12, 1928 the dam failed catastrophically and the resulting flood killed up to 600 people. The collapse of the St. Francis Dam is considered to be one of the worst American civil engineering disasters of the 20th century and remains the second-greatest loss of life in California's history (after the 1906 San Francisco Earthquake and fire). The disaster marked the end of Mulholland's career.

The failure of the dam is now believed to have begun when the eastern abutment of the dam gave way due to a landslide. The material on which the eastern abutment of the dam had been built was itself part of an ancient landslide, but this would have been impossible for almost any geologist of the 1920s to detect.

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